High-frequency detection method and high-frequency detection circuit

ABSTRACT

A high frequency detection circuit detects information about a first high frequency power in a high frequency power source device supplying the first high frequency power having a first frequency and a second high frequency power having a second frequency lower than the first frequency to a load. A third high frequency signal that is a mixed signal of a first high frequency signal having the first frequency and a second high frequency signal having the second frequency is detected by a directional coupler. The third high frequency signal is converted to a fourth high frequency signal having a third frequency between the first and second frequencies using a heterodyne system. A progressive wave power of the first frequency is detected based on the fourth high frequency signal.

TECHNICAL FIELD

The present invention relates to a high frequency detection method and ahigh frequency detection circuit, and more particularly to a highfrequency detection method and a high frequency detection circuit fordetecting information about a first high frequency power in a highfrequency power source device supplying the first high frequency powerhaving a first frequency and a second high frequency power having asecond frequency lower than the first frequency to a load.

BACKGROUND ART

In recent years, there has been developed a plasma process technique forsupplying a high frequency power at a relatively high frequency f and ahigh frequency power at a relatively low frequency f1 to a reactionchamber. In this plasma process technique, plasma is generated mainlywith the high frequency power at frequency f, and the high frequencypower at frequency f1 is used to control the behavior of ions in thevicinity of a substrate.

In such a plasma process technique, it is important to accuratelymeasure the voltage, current, power, and the like of frequency f. When fis approximately 1000 times higher than frequency f1, however, themeasurement is difficult. This is because the high frequency power atfrequency f is modulated by the high frequency power at frequency f1 inthe reaction chamber to cause a high frequency power at a frequencyf±nf1 (where n is an integer equal to or greater than 0), and a filtercircuit that extracts only the high frequency signal at frequency f fromthe high frequency signal at frequency f±nf1 is not available atpresent.

Therefore, the voltage, current, power and the like of frequency f±nf1has conventionally been used as the voltage, current, power, and thelike of frequency f. Accordingly, the level variations of the voltage,current, power, and the like of frequency f1 also causes the levelvariations of the voltage, current, power, and the like of frequency f,thereby leading to a low reproducibility in measurement.

DISCLOSURE OF THE INVENTION

The present invention aims to provide a high frequency detection methodand a high frequency detection circuit capable of easily and accuratelydetecting information about a first high frequency power in a highfrequency power source device supplying the first frequency power havinga first frequency and a second high frequency power having a secondfrequency lower than the first frequency to one load.

In a high frequency detection method in accordance with the presentinvention, the following steps are performed: a first step of detectinga third high frequency signal that is a mixed signal of a first highfrequency power having a first frequency and a second high frequencypower having a second frequency at a prescribed node of a high frequencypower source device; a second step of generating a reference signalhaving a frequency shifted toward a higher frequency side or a lowerfrequency side from the first frequency by a third frequency between thefirst and second frequencies; a third step of generating a mixed signalof the third high frequency signal detected at the first step and thereference signal generated at the second step; a fourth step ofextracting a fourth high frequency signal having the third frequencyfrom the mixed signal generated at the third step; and a fifth step ofdetecting information about the first high frequency power based on thefourth high frequency signal extracted at the fourth step. Therefore,the third high frequency signal that is a mixed signal of the first highfrequency signal at the first frequency and the second high frequencysignal at the second frequency is converted to the fourth high frequencysignal at the third frequency between the first and second frequenciesusing a heterodyne system, and based on the fourth high frequencysignal, the information about the first high frequency power isdetected, so that the information about the first high frequency powercan be detected easily and accurately and the high frequency powersource device can be controlled accurately.

A high frequency detection circuit in accordance with the presentinvention is provided with: a signal detection circuit detecting a thirdhigh frequency signal that is a mixed signal of a first high frequencypower having a first frequency and a second high frequency power havinga second frequency at a prescribed node of a high frequency power sourcedevice; a signal generation circuit generating a reference signal havinga frequency shifted toward a higher frequency side or a lower frequencyside from the first frequency by a third frequency between the first andsecond frequencies; a mixer circuit generating a mixed signal of thethird high frequency signal detected by the signal detection circuit andthe reference signal generated by the signal generation circuit; afilter circuit extracting a fourth high frequency signal having thethird frequency from the mixed signal generated by the mixer circuit;and an information detection circuit detecting information about thefirst high frequency power based on the fourth high frequency signalextracted by the filter circuit. Therefore, the third high frequencysignal that is a mixed signal of the first high frequency signal at thefirst frequency and the second high frequency signal at the secondfrequency is converted to the fourth high frequency signal at the thirdfrequency between the first and second frequencies using a heterodynesystem, and based on the fourth high frequency signal, the informationabout the first high frequency power is detected, so that theinformation about the first high frequency power can be detected easilyand accurately and the high frequency power source device can becontrolled accurately.

Preferably, two sets of the signal detection circuits, the signalgeneration circuits, the mixer circuits, and the filter circuits areprovided. The signal detection circuit in one set detects the third highfrequency signal indicative of a high frequency voltage at theprescribed node. The signal detection circuit in the other set detectsthe third high frequency signal indicative of a high frequency currentat the prescribed node. The information detection circuit detects theinformation about the first high frequency power based on the two fourthhigh frequency signals extracted by the two filter circuits. In thiscase, it is possible to detect a high frequency voltage, a highfrequency current, a ratio therebetween, a phase differencetherebetween, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a semiconductormanufacturing apparatus in accordance with an embodiment of the presentinvention.

FIG. 2 is a block diagram showing a configuration of a high frequencypower source 1 shown in FIG. 1.

FIG. 3 is a circuit diagram showing a configuration of a directionalcoupler shown in FIG. 2.

FIG. 4 is a block diagram showing a configuration of a filter circuitshown in FIG. 2.

FIG. 5 is a spectral diagram illustrating an operation of the filtercircuit shown in FIG. 4.

FIG. 6 is a circuit block diagram showing a configuration of a highfrequency sensor 2 shown in FIG. 1.

FIG. 7 is a block diagram showing a configuration of a matching unit 3shown in FIG. 1.

FIG. 8 is a diagram showing a configuration of a reaction chamber shownin FIG. 1.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 is a block diagram showing a configuration of a semiconductormanufacturing apparatus in accordance with an embodiment of the presentinvention. In FIG. 1, the semiconductor manufacturing apparatus includeshigh frequency power sources 1, 9, high frequency sensors 2, 5, matchingunits 3, 8, a controller 4, a monitoring device 6, and a reactionchamber 7. The semiconductor manufacturing apparatus supplies a highfrequency power having a relatively high frequency f (or example 500MHz) from high frequency power source 1 and a high frequency powerhaving a relatively low frequency f1 (for example 800 KHz) from highfrequency power source 9 to reaction chamber 7 to generate plasma.

High frequency power source 1 is a main power source device forsupplying the high frequency power at frequency f to reaction chamber 7.High frequency power source 1 includes an oscillator, 10, an amplifier11, a directional coupler 12, filter circuits 13, 16, an output powerdetector 14, an output power display portion 15, a reflected powerdetector 17, and a reflected power display portion 18, as shown in FIG.2.

Oscillator 10 outputs a high frequency signal at frequency f. Amplifier11 amplifies the high frequency signal generated at oscillator 10 to asignal of a desired power. The amplified high frequency signal issupplied through directional coupler 12 to matching unit 3.

Directional coupler 12 includes a primary coaxial line 20, a secondarycoaxial line 21, and resistance elements 22, 23, as shown in FIG. 3.Primary coaxial line 20 includes an internal conductor 20 a and anexternal conductor 20 b and forms a part of the coaxial line betweenamplifier 11 and matching unit 3. Secondary coaxial line 21 includes aninternal conductor 21 a and an external conductor 21 b. Externalconductors 20 b, 21 b are grounded together. The middle portion ofsecondary coaxial line 21 is loosely coupled to a part of primarycoaxial line 20 in such a manner that an impedance change of primarycoaxial line 20 is as small as possible. Resistance element 22 isconnected between an upstream end portion of internal conductor 21 a ofsecondary coaxial line 21 and a ground potential GND line. Resistanceelement 23 is connected between a downstream end portion of internalconductor 21 a of secondary coaxial line 21 and a ground potential GNDline. Resistance elements 22 and 23 have the same resistance value R.

A characteristic impedance Z₀ of primary coaxial line 20, a couplingcapacitance C₀ between primary coaxial line 20 and secondary coaxialline 21, a mutual inductance M between primary coaxial line 20 andsecondary coaxial line 21, and resistance value R of resistance elements22, 23 have a relationship represented by Z₀=M/C₀R. Where the voltagesacross terminals of resistance elements 22, 23 are V1 and V2,respectively, and a proportionality constant is K, a progressive wavepower Pf and a reflected wave power Pr are represented by K(V1)² andK(V2)², respectively. Output voltages V1, V2 of directional coupler 12are applied to filer circuits 13, 16, respectively.

Filter circuit 13 converts a high frequency signal V1 having a frequencyf=nf1 to a high frequency signal V1′ having a frequency Δf, using aheterodyne system. Frequency Δf is a frequency (for example 10.6 MHz)which is sufficiently lower than frequency f of high frequency powersource 1 and higher than frequency f1 of high frequency power source 9.More specifically, filter circuit 13 includes a local oscillator 25, amixer 26, and a bandpass filter (BPF) 27, as shown in FIG. 4. Localoscillator 25 generates a high frequency signal VR having a frequencyf+Δf, which is a sum of frequency f of high frequency power source 1 anda prescribed frequency Δf, for application to mixer 26. Mixer 26 mixesoutput signal V1 of directional coupler 12 with output signal VR oflocal oscillator 25.

Here, V1 has a mixed wave formed of the output signals of two highfrequency power sources 1, 9 and has a component of frequency f±nf1, asshown in FIG. 5. An output signal V1×VR of mixer 26 has three componentsof a center frequency f±nf1 and frequencies Δf±nf1, 2 f+Δnf1. It isdifficult to create a bandpass filter for extracting the component offrequency f from signal V1 having the component of frequency f+nf1, asf/f1 is approximately 1000. However, it is possible to create a bandpassfilter for extracting the component of frequency Δf from signal V1×VRhaving the component of frequency Δf±nf1, as Δf/f1 is approximately 10.Bandpass filter 27 extracts signal component V1′ at frequency Δf fromsignal V1×VR. The level information and phase information of signal V1′includes the level information and phase information of the frequency fcomponent of signal V1.

Output power detector 14 detects level information V (for example anamplitude) of signal V1′ and detects progressive wave power Pf atfrequency f based on a formula Pf=K′V² (where K′ is a proportionalityconstant). Output power display portion 15, for example, digitallydisplays progressive wave power Pf detected by output power detector 14.

Filter circuit 16 also has the same configuration as filter circuit 13.A signal V2′ including the level information and phase information ofthe component of frequency f of signal V2 is applied from filter circuit16 to reflected power detector 17. Reflected power detector 17 detectslevel information V of signal V2′ and detects reflected wave power Pr atfrequency f based on the formula Pr=K′V². Reflected power displayportion 18, for example, digitally displays reflected wave power Prdetected by reflected power detector 17.

High frequency sensor 2 includes a coaxial line 30, coils 31, 32,capacitors 33–44, diodes 45–48, resistance elements 49–57, variableresistance elements 58, 59, and filter circuits 60–63, as shown in FIG.6. Each of filter circuits 60–63 has the same configuration as filtercircuit 13 shown in FIG. 4. Coaxial line 30 includes an internalconductor 30 a and an external conductor 30 b and forms a part of thecoaxial line between high frequency power source 1 and matching unit 3.

Coil 31 is inductively coupled to internal conductor 30 a of coaxialline 30, and a current I having a level and a phase corresponding to thecurrent flowing in internal conductor 30 a flows in coil 31. Oneterminal of coil 31 is connected to an input node 60 a of filter circuit60. Capacitors 33, 34 are connected in series between internal conductor30 a and an input node 61 a of filter 61. A voltage V having a level anda phase corresponding to the voltage of internal conductor 30 a occursat node 61 a. Diode 46, resistance element 52, variable resistanceelement 58, resistance element 49, and diode 45 are connected in seriesbetween an output node 61 b of filter circuit 61 and an output node 60 bof filter circuit 60. A sliding terminal 58 a of variable resistanceelement 58 is fixed at a prescribed position. A signal VZ correspondingto a shift with respect to characteristic impedance Z₀ (for example 50Ω) of a ratio Z between voltage V and current I appears at terminal 58a. The level of signal VZ is 0 at the time of matching.

Coil 32 is inductively coupled to internal conductor 30 a of coaxialline 30, and current I having a level and a phase corresponding to thecurrent flowing in internal conductor 30 a flows in coil 32. Twoterminals of coil 32 are connected to input nodes 62 a, 63 a of filtercircuits 62, 63, respectively. Capacitors 39, 40 are connected in seriesbetween internal conductor 30 a and a node N54. Voltage V having a leveland a phase corresponding to the voltage of internal conductor 30 aoccurs at node N54. Node N54 is connected to two terminals of coil 32through resistance elements 54, 56 and is also grounded throughresistance element 55.

Diode 47, resistance element 53, variable resistance element 59,resistance element 57, and diode 48 are connected in series between anoutput node 62 a of filter circuit 62 and an output node 63 a of filtercircuit 63. A sliding terminal 59 a of variable resistance element 59 isfixed at a prescribed position. Resistance elements 54–56 and diodes 47,48 constitute a balance module. A signal Vφ having a level correspondingto a phase difference φ between voltage V and current I appears at aterminal 59 a. The level of signal Vφ is 0 at the time of matching. Itis noted that capacitors 35–38, 41–44 and resistance elements 49, 52,53, 57 are provided for noise elimination and potential smoothing.

Matching unit 3 includes matching elements 64, 65 and driving devices66, 67, as shown in FIG. 7. The impedance of each of matching elements64, 65 is controllable. When the impedance as seen from the inputterminal side of matching unit 3 to the reactor 7 side becomes equal tocharacteristic impedance Z₀ of the coaxial line between high frequencypower source 1 and matching unit 3, reflected power Pr has the minimumvalue. Each of driving devices 66, 67 includes a motor, a gear, and thelike to drive each of matching units 64, 65.

Controller 4 adjusts the impedance of matching element 64 throughdriving device 66 such that the level of signal VZ from high frequencysensor 2 attains the minimum value, and it also adjusts the impedance ofmatching element 65 through driving unit 67 such that the level ofsignal Vφ from high frequency sensor 2 attains the minimum value.

Returning to FIG. 1, high frequency sensor 5 detects voltage V andcurrent I on the interconnection between matching unit 3 and reactionchamber 7. High frequency sensor 5 includes, for example, coil 31,capacitors 33, 34, resistance element 50, and filter circuits 60, 61 ofhigh frequency sensor 2 shown in FIG. 6. Output nodes 60 b, 61 b offilter circuits 60, 61 are connected to monitoring device 6. Monitoringdevice 6 is formed, for example, of an oscilloscope. Monitoring device 6is capable of monitoring a voltage, a current, and impedance between theelectrodes of reaction chamber 7 and detecting the state of plasmabetween the electrodes.

High frequency power source 9 is a bias power source device forsupplying the high frequency power at frequency f1 to reaction chamber7. Matching unit 8 is provided in order to limit the reflected wavepower to high frequency power source 9 to the minimum value. Matchingunit 8 includes a coil and a capacitor and performs a function ofpreventing the high frequency power from high frequency power source 1from being input to high frequency power source 9.

As shown in FIG. 8, for example, two parallel flat plate electrodes 71,72 are provided within reaction chamber 7. The high frequency powersfrom high frequency power sources 1, 9 are supplied to electrodes 71,72, respectively. A substrate 73 is set on the surface of electrode 72.

In etching and film deposition, the air in reaction chamber 7 is firstexhausted by a vacuum pump (not shown). Then, a prescribed gas isintroduced into reaction chamber 7 at a prescribed flow rate, and anexhausting rate of the vacuum pump is adjusted so that the pressure inreaction chamber 7 is adjusted at a prescribed value.

Then, high frequency power sources 1, 9 are turned on to supplyprescribed high frequency powers to reaction chamber 7. As a result, thegas between electrodes 71 and 72 is ionized in a plasma state. The powerfor bringing the gas into the plasma state is mainly supplied from highfrequency power source 1, while the power for applying the gas ions tosubstrate 73 is mainly supplied from high frequency power source 9. Whenetching gas (for example CF₄) is used, the surface of substrate 73 isetched. When film depositing gas (for example SiH₄) is used, a film isdeposited on the surface of substrate 73.

In the present embodiment, a mixed signal of a high frequency signalhaving frequency f and a high frequency signal having frequency f1 isconverted into a high frequency signal having frequency Δf(f>Δf>f1)using the heterodyne system, and based on this high frequency signal,the high frequency voltage, the current, the ratio therebetween, thephase difference therebetween, the power, and the like of frequency fare detected. It is therefore possible to easily and accurately detectthe high frequency voltage and the like of frequency f and to accuratelycontrol the high frequency power.

It is noted that although in the present embodiment, local oscillator 25generates high frequency signal VR having frequency f+Δf, which is a sumof frequency f of high frequency power source 1 and a prescribedfrequency Δf, for application to mixer 26, it is needless to say thatthe same result is obtained when local oscillator 25 generates a highfrequency signal VR having a frequency f−Δf, which is a differencebetween f and Δf. Note that in this case, output signal V1×VR of mixer26 has three components: center frequency f±nf1 and frequencies Δf±nf1,2f−Δf±nf1.

It should be understood that the embodiment disclosed herein is taken byway of illustration not by way of limitation in all the respects. Thescope of the present invention is shown not in the forgoing descriptionbut in the claims, and it is intended that all equivalents to the claimsand all modifications within the claims should be embraced.

1. A high frequency detection method for detecting information about afirst high frequency power in a plasma generator, for generating plasma,by supplying said first high frequency power having a first frequency toa reaction chamber via a first power supply line and supplying a secondhigh frequency power having a second frequency lower than said firstfrequency to said reaction chamber via a second power supply line, saidfirst high frequency power being modulated by said second high frequencypower to generate a third high frequency power having a third frequencyin said reaction chamber, said high frequency detection methodcomprising: a first step of detecting a first high frequency signalhaving components of said first and third frequencies propagatingthrough said first power supply line; a second step of generating asecond high frequency signal having a frequency which is a sum of saidfirst frequency and a predetermined fourth frequency between said firstand second frequencies, or said second high frequency having a frequencywhich is a difference between said first frequency and the predeterminedfourth frequency which is between said first and second frequencies; athird step of generating a third high frequency signal by mixing saidfirst and second high frequency signals; a fourth step of extracting afourth high frequency signal having said fourth frequency from saidthird high frequency signal; and a fifth step of obtaining theinformation about said first high frequency power based on said fourthhigh frequency signal.
 2. A high frequency detection circuit fordetecting information about a first high frequency power in a plasmagenerator, for generating plasma, by supplying said first high frequencypower having a first frequency to a reaction chamber via a first powersupply line and supplying a second high frequency power having a secondfrequency lower than said first frequency to said reaction chamber via asecond power supply line, said first high frequency power beingmodulated by said second high frequency power to generate a third highfrequency power having a third frequency in said reaction chamber, saidhigh frequency detection circuit comprising: a signal detection circuitdetecting a first high frequency signal having components of said firstand third frequencies propagating through said first power supply line;a local oscillator generating a second high frequency signal having afrequency which is a sum of said first frequency and a predeterminedfourth frequency which is between said first and second frequencies, orsaid second high frequency signal being a difference between said firstfrequency and the predetermined fourth frequency which is between saidfirst and second frequencies; a mixer circuit generating a third highfrequency signal by mixing said first and second high frequency signals;a filter circuit extracting a fourth high frequency signal having saidfourth frequency from said third high frequency signal; and a detectorobtaining the information about said first high frequency power based onsaid fourth high frequency signal.
 3. A high frequency detection circuitfor detecting a high frequency power in a plasma generator, forgenerating plasma, by supplying a first high frequency power having afirst frequency to a reaction chamber via a first power supply line andsupplying a second high frequency power having a second frequency lowerthan said first frequency to said reaction chamber via a second powersupply line, said first high frequency power being modulated by saidsecond high frequency power to generate a third high frequency powerhaving a third frequency in said reaction chamber, said high frequencydetection circuit comprising: a directional coupler coupled to a portionof said first power supply line and detecting a first high frequencysignal indicative of a progressive wave power progressing to saidreaction chamber and having components of said first and thirdfrequencies, and a second high frequency signal indicative of areflected wave power reflected from said reaction chamber and having thecomponents of said first and third frequencies; a local oscillatorgenerating a third high frequency signal having a frequency which is asum of said first frequency and a predetermined fourth frequency betweensaid first and second frequencies, or a difference between said firstfrequency and the predetermined fourth frequency between said first andsecond frequencies; a first mixer circuit generating a fourth highfrequency signal by mixing said first and third high frequency signals;a second mixer circuit generating a fifth high frequency signal bymixing said second and third high frequency signals; a first filtercircuit extracting a sixth high frequency signal having said fourthfrequency from said fourth mixed signal; a second filter circuitextracting a seventh high frequency signal having said fourth frequencyfrom said fifth mixed signal; a first power detector obtaining thecomponent of said first frequency in the progressive wave powerprogressing to said reaction chamber based on said sixth high frequencysignal; and a second power detector obtaining the component of saidfirst frequency in the reflected wave power reflected from said reactionchamber based on said seventh high frequency signal.
 4. A high frequencydetection circuit for detecting a matching state in a plasma generator,for generating plasma, by supplying a first high frequency power havinga first frequency to a reaction chamber via a first power supply lineand a first matching unit and supplying a second high frequency powerhaving a second frequency lower than said first frequency to saidreaction chamber via a second power supply line and a second matchingunit, said first high frequency power being modulated by said secondhigh frequency power to generate a third high frequency power having athird frequency in said reaction chamber, said high frequency detectioncircuit comprising: a signal detection circuit coupled to a portion ofsaid first power supply line and detecting a first high frequency signalindicative of a current flowing through said first power supply line andhaving components of said first and third frequencies, and a second highfrequency signal indicative of a voltage of said first power supply lineand having the components of said first and third frequencies; a localoscillator generating a third high frequency signal having a frequencywhich is a sum of said first frequency and a predetermined fourthfrequency between said first and second frequencies, or a differencebetween said first frequency and the predetermined fourth frequencybetween said first and second frequencies; a first mixer circuitgenerating a fourth high frequency signal by mixing said first and thirdhigh frequency signals; a second mixer circuit generating a fifth highfrequency signal by mixing said second and third high frequency signals;a first filter circuit extracting a sixth high frequency signal havingsaid fourth frequency from said fourth mixed signal; a second filtercircuit extracting a seventh high frequency signal having said fourthfrequency from said fifth mixed signal; and a signal generating circuitoutputting a signal indicative of a matching state of said first powersupply line and said reaction chamber based on said sixth and seventhhigh frequency signals.
 5. A high frequency detection circuit fordetecting a high frequency current in a plasma generator, for generatingplasma, by supplying a first high frequency power having a firstfrequency to a reaction chamber via a first power supply line andsupplying a second high frequency power having a second frequency lowerthan said first frequency to said reaction chamber via a second powersupply line, said first high frequency power being modulated by saidsecond high frequency power to generate a third high frequency powerhaving a third frequency in said reaction chamber, said high frequencydetection circuit comprising: a signal detection circuit coupled to aportion of said first power supply line and detecting a first highfrequency signal indicative of a current flowing through said firstpower supply line and having components of said first and thirdfrequencies; a local oscillator generating a second high frequencysignal having a frequency which is a sum of said first frequency and apredetermined fourth frequency between said first and secondfrequencies, or a different between said first frequency and thepredetermined fourth frequency between said first and secondfrequencies; a mixer circuit generating a third high frequency signal bymixing said first and second high frequency signals; and a filtercircuit extracting a fourth high frequency signal having said fourthfrequency and indicative of the component of said first frequency in thecurrent flowing through said first power supply line from said thirdhigh frequency signal.
 6. A high frequency detection circuit fordetecting a high frequency voltage in a plasma generator for generatingplasma by supplying a first high frequency power having a firstfrequency to a reaction chamber via a first power supply line andsupplying a second high frequency power having a second frequency lowerthan said first frequency to said reaction chamber via a second powersupply line, said first high frequency power being modulated by saidsecond high frequency power to generate a third high frequency powerhaving a third frequency in said reaction chamber, said high frequencydetection circuit comprising: a signal detection circuit coupled to aportion of said first power supply line and detecting a first highfrequency signal indicative of a voltage of said first power supply lineand having components of said first and third frequencies; a localoscillator generating a second high frequency signal having a frequencywhich is a sum of said first frequency and a predetermined fourthfrequency between said first and second frequencies, or a differencebetween said first frequency and the predetermined fourth frequencybetween said first and second frequencies; a mixer circuit generating athird high frequency signal by mixing said first and second highfrequency signals; and a filter circuit extracting a fourth highfrequency signal having said fourth frequency and indicative of thecomponent of said first frequency in the voltage of said first powersupply line.